The present invention relates to turbochargers, and more particularly relates to two-stage turbocharger systems.
The performance of internal combustion engines can be enhanced with the use of a turbocharger for increasing the pressure of the air supplied to the engine intake. In a simple turbocharger system, a single turbocharger is coupled with the engine. The turbocharger includes a turbine driven by exhaust gas from the engine exhaust manifold, and a compressor driven by the turbine for compressing air before it is delivered to the engine intake. The increased pressure of the intake air enables the engine to produce a greater amount of power.
In many engine systems, a single turbocharger is adequate. In other cases, however, it is desirable to provide maximum boost pressures that cannot readily be achieved with a single turbocharger. Accordingly, it is known to employ two turbochargers arranged in series. A low-pressure turbocharger is arranged in series with a high-pressure turbocharger, such that engine intake air is first compressed in the low-pressure compressor and is then further compressed in the high-pressure compressor. Exhaust gas from the engine is supplied to the high-pressure turbine for driving the high-pressure compressor, and to the low-pressure turbine for driving the low-pressure compressor. Typically the turbines are connected in series but a bypass passage is provided for bypassing the high-pressure turbine under certain operating conditions. A bypass valve is employed for selectively opening the bypass passage to bypass the high-pressure turbine, in which case the system operates as a single-stage system using only the low-pressure turbocharger to provide engine boost, or closing the bypass passage such that the two turbochargers operate in series to provide a higher level of boost.
The bypass valve in conventional two-stage turbocharger systems often is housed in the turbine housing of one of the turbochargers, or in a separate housing arranged between the engine exhaust manifold and the turbochargers. A separate duct is also provided for connecting the exhaust gas outlet of the high-pressure turbine housing to the exhaust gas inlet of the low-pressure turbine housing. Such arrangements become mechanically complex and entail multiple connections between components that are subject to leakage at flange joints. Special and costly sealing arrangements are often required to attempt to eliminate or reduce such leakage.